ch-run: run SIF files

[kchilleri]

ch-run can now identify, validate, and run SIF files (see: https://github.com/sylabs/sif/tree/main).
ch-run [OPTION...] IMAGE -- COMMAND [ARG...]
When given an image, we will identify and validate the file by looking through the SIF header and obtaining the SIF_MAGIC constant. This constant confirms it is a SIF file.

Since a SIF file is a squash image with some extra metadata (header and signatures at end of file), we need to find the byte offset of the squash portion to feed to ch_fuse.c and continue the usual squash mount/run.

Comparison example:

kchilleri@dev:/usr/local/src/charliecloud$ ch-run --verbose /home/kchilleri/charliecloud_dev/sif_files/alpine.sqfs -- true
ch-run[49244]: squash image file magic expected: 6873 7173; actual: 6873 7173 (ch_core.c:413)
ch-run[49244]: verbosity: 1 (ch-run.c:239)
ch-run[49244]: image: /home/kchilleri/charliecloud_dev/sif_files/alpine.sqfs (ch-run.c:240)
ch-run[49244]: storage: /var/tmp/kchilleri.ch (ch-run.c:241)
ch-run[49244]: newroot: (null) (ch-run.c:242)
ch-run[49244]: container uid: 1000 (ch-run.c:243)
ch-run[49244]: container gid: 1000 (ch-run.c:244)
ch-run[49244]: join: 0 0 (null) 0 (ch-run.c:245)
ch-run[49244]: private /tmp: 0 (ch-run.c:247)
ch-run[49244]: seccomp: 0 (ch-run.c:249)
ch-run[49244]: unsafe: 0 (ch-run.c:251)
ch-run[49244]: using default mount point: /var/tmp/kchilleri.ch/mnt (ch_fuse.c:127)
ch-run[49244]: Squash byte offset: 0
 (ch_fuse.c:240)
ch-run[49244]: Magic number at this position: 6873 7173
 (ch_fuse.c:241)
ch-run[49245]: executing: true (ch_core.c:586)
ch-run[49244]: FUSE loop terminated successfully (ch_fuse.c:187)
ch-run[49244]: child terminated normally with exit code 0 (ch_fuse.c:196)
ch-run[49244]: unmounting: /var/tmp/kchilleri.ch/mnt (ch_fuse.c:212)
ch-run[49244]: FUSE loop done (ch_fuse.c:216)

kchilleri@dev:/usr/local/src/charliecloud$ ch-run --verbose /home/kchilleri/charliecloud_dev/sif_files/alpine.sif -- true
ch-run[49261]: squash image file magic expected: 6873 7173; actual: 2321 2f75 (ch_core.c:413)
ch-run[49261]: sif file magic expected: SIF_MAGIC: actual: SIF_MAGIC (ch_core.c:430)
ch-run[49261]: SIF file detected! (ch-run.c:212)
ch-run[49261]: verbosity: 1 (ch-run.c:239)
ch-run[49261]: image: /home/kchilleri/charliecloud_dev/sif_files/alpine.sif (ch-run.c:240)
ch-run[49261]: storage: /var/tmp/kchilleri.ch (ch-run.c:241)
ch-run[49261]: newroot: (null) (ch-run.c:242)
ch-run[49261]: container uid: 1000 (ch-run.c:243)
ch-run[49261]: container gid: 1000 (ch-run.c:244)
ch-run[49261]: join: 0 0 (null) 0 (ch-run.c:245)
ch-run[49261]: private /tmp: 0 (ch-run.c:247)
ch-run[49261]: seccomp: 0 (ch-run.c:249)
ch-run[49261]: unsafe: 0 (ch-run.c:251)
ch-run[49261]: using default mount point: /var/tmp/kchilleri.ch/mnt (ch_fuse.c:127)
ch-run[49261]: Squash byte offset: 262144
 (ch_fuse.c:240)
ch-run[49261]: Magic number at this position: 6873 7173
 (ch_fuse.c:241)
ch-run[49262]: executing: true (ch_core.c:586)
ch-run[49261]: FUSE loop terminated successfully (ch_fuse.c:187)
ch-run[49261]: child terminated normally with exit code 0 (ch_fuse.c:196)
ch-run[49261]: unmounting: /var/tmp/kchilleri.ch/mnt (ch_fuse.c:212)
ch-run[49261]: FUSE loop done (ch_fuse.c:216)

closes #1749

[kchilleri]

To Do:

1. Note: this is a draft. Need to do more error checking.
2. Add documentation.
3. Concern: Do I need to worry about SIF files sometimes having signature blocks at the end? Since we feed the byte offset of when the squash data begins but not when it ends.