service: add workdir support for daemon instances

[1715173329]

Allow instances to select a working directory with the workdir option.
Change directory after setting the process identity and fail startup
if the directory cannot be entered. Preserve the existing behavior
when workdir is omitted.

Pass the directory to ujail through a new -W option, reusing its
existing directory switch after jail setup. Track workdir changes,
include the value in instance dumps, and release owned strings.

[1715173329]

Kindly ping @dangowrt @PolynomialDivision

[dangowrt]

Thanks for the workdir option, it is a welcome addition. In the meantime a larger series landed on main, and the PR needs a few changes before it can go in:

1. Option letter. -W <file> is now taken in ujail by the resources override (5beb80f), and procd passes -W for the jail resources attribute (26a0309). Merged as is, procd would hand the working directory to ujail as a resources file. Please use one of the letters still free in OPT_ARGS: B, H, L, Q, v or z.
2. Rebase. The PR no longer applies to main (conflicts in jail/jail.c). main() and its getopt loop now sit at the end of jail.c, and OPT_ARGS and the usage text have changed, so the hunks need to move along with them.
3. procd.sh. _procd_set_param() in package/system/procd/files/procd.sh (openwrt/openwrt) only forwards the keys it knows, so procd_set_param workdir /some/dir in an init script is silently dropped. Please open the matching openwrt/openwrt PR adding workdir to the string parameters and link it here, so that both can be merged together.

For bundle (OCI) instances, process.cwd from the bundle still takes precedence because parseOCIprocess() runs after option parsing. That looks right to me, and a sentence about it in the commit message would help.

[1715173329]

Rebased upon master branch.

3. procd.sh. _procd_set_param() in package/system/procd/files/procd.sh (openwrt/openwrt) only forwards the keys it knows, so procd_set_param workdir /some/dir in an init script is silently dropped. Please open the matching openwrt/openwrt PR adding workdir to the string parameters and link it here, so that both can be merged together.

procd.sh depends on this PR so I plan to do it after this get merged.