Fix getmntpoint(3) to operate as it is documented in its manual page.

[mckusick]

Summary

Dag-Erling Smørgrav <[e-mail]> reoprts:
While working on quot(8), I noticed that getmntpoint(3) does not actually accept a device name “with or without /dev/ prepended to it” as the manual page and code comments claim.

This patch corrects the problem by prepending /dev/ to names that do not begin with a '/'.

Since there are now two places in the code that want to prepend /dev/ the code to do this is broken out into a separate function.

Test Plan

Note that this fix cannot be tested with the df(1) command as it does not use getmntpoint(3).

This program can be used to test getmntpoit(3): {F132597285}

[des]

lib/libutil/mntopts.c

151

Please use a caller-provided buffer instead

179

Missing a null check here

[mckusick]

Respond to Dag-Erling's comments.

[mckusick]

Responding to Dag-Erling's comments.

lib/libutil/mntopts.c

151

It takes more parameters, more lines of code, and generally makes the code harder to follow. That said, I have made the change.

179

stat(NULL, &statbuf) returns -1 (EBADF) which will have the correct response here. It seems pointless to add an explicit test for NULL.