src/fs.{cc,h}: Update fs_stats without error spam.

[pghvlaans]

Checklist

◯　I have described the changes
◯　I have linked to any relevant GitHub issues, if applicable
Documentation in doc/ has been updated (N/A)
◯　All new code is licensed under GPLv3

Description

• Describe the changes, why they were necessary, etc
  RE 788ac88, "conky: statfs64 '/media/usbstick': No such file or directory" spam in ~/.xsession-errors #208

update_fs_stats was recently altered to clear set for all file systems every thirteen seconds, and its update_fs_stat call was removed. This was done to mitigate a longstanding issue whereby repeated errors appeared when updates were attempted for a nonexistent mountpoint. However, before the change, update_fs_stat was only called from prepare_fs_stat and update_fs_stats. In consequence, update_fs_stat is now called for each filesystem only at init time, and no changes are displayed.

Updating capabilities can be restored by reverting the change to update_fs_stats. In addition, repeated error messages can be suppressed in the following manner:

1. Add a flag called errored and initialize to zero.
2. If a mountpoint is not found, write an error only if errored is zero; once this has been done, set errored.
3. Once the mountpoint has been found again, clear errored so that a new message can appear.

• Describe how the changes will affect existing behaviour.

Filesystem information will be updated periodically as before. Nonexistent mountpoints now produce errors only once at init, and once more if an existing mountpoint is removed. That is, errors will no longer be produced at every update.

• Describe how you tested and validated your changes.

I ran patched conky from a terminal to monitor standard error using a config file that included a spurious mountpoint. I periodically made and deleted that directory, verifying that error messages appeared only once upon deletion. This worked both with and without if_mounted. Live updates to fs_used and fs_free occurred as expected.

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