restore sort function pointers when restoring cfg

[azuline]

closes #1757

This PR fixes a sorting synchronization issue.

The root cause of the bug in #1757 is that the sorting function entrycmpfn is not restored when changing contexts, causing reverse (and version) to be "global" in that setting them on one context caused them to be set on all contexts.

This PR fixes the bug by setting entrycmpfn and namecmpfn to the correct pointers whenever cfg is set.

---

simulation of state variables in the bugged scenario:

1. starting state
   • cfg0.reverse = 0, entrycmpfn = &entrycmp
2. set context 1 to r
   • cfg0.reverse = 1, entrycmpfn = &reventrycmp
3. create context 2. context 2 should default to r.
   • cfg0.reverse = 1, cfg1.reverse = 1, entrycmpfn = &reventrycmp
4. unset r in context 2.
   • cfg0.reverse = 1, cfg1.reverse = 0, entrycmpfn = &entrycmp
5. switch back to context 1. context 1 should not appear to be r.
   • cfg0.reverse = 1, cfg1.reverse = 0, entrycmpfn = &entrycmp
6. try to set r again. no-ops when it should do something.
   • cfg0.reverse = 0, cfg1.reverse = 0, entrycmpfn = &entrycmp

the "double unset" behavior is because the statusbar uses a function pointer comparison to print R and V. it does not use cfg.reverse or cfg.version. if the statusbar used cfg.reverse, we would see R in the statusbar, yet no reverse sort in the entrylist.

video :)

nnn-bugfix.mp4

[jarun]

Thank you!