Update rc.initdiskless, fix error handling of remount_optional

chkerr() ignores the exit code of a preceding mount command in case a
file ```remount_optional``` exists. The check is performed and a

Update rc.initdiskless, fix error handling of remount_optional

chkerr() ignores the exit code of a preceding mount command in case a
file ```remount_optional``` exists. The check is performed and a
subshell is launched to log the informational message and return. The
return is executed in the context of the subshell, not the context of
the chkerr() function, hence is a NOP. The remount_optional check is
hence ineffective.

Change the code to if/then/fi, so the return is evaluated in the context
of the chkerr function, to make the check effective.

Reviewed by: imp, emaste
Pull Request: https://github.com/freebsd/freebsd-src/pull/1497

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rc: Document rtadvd_flags

PR: 283696
MFC after: 1 week