Fix for re-reading /etc/mtab, part 2.

[jmlayman]

This is a continuation of fb5c53e:

When /etc/mtab is updated on Linux it's done atomically with
rename(2).  A new mtab is written, the existing mtab is unlinked,
and the new mtab is renamed to /etc/mtab.  This means that we
must close the old file and open the new file to get the updated
contents.  Using rewind(3) will just move the file pointer back
to the start of the file, freopen(3) will close and open the file.

In this commit, a few more rewind(3) calls were replaced with freopen(3)
to allow updated mtab entries to be picked up immediately.

Note that technically only libzfs_mount.c & libzfs_util.c need to be modified as they are libraries and so the functions within them can be called multiple times (during which time /etc/mtab could change). Since zfs_main.c is a utility front-end, it probably isn't necessary to modify it but I went ahead & did so anyway. If this is not desired, those changes can be omitted.

[behlendorf]

Do you have a specific use case where you were seeing a stale /etc/mtab?

[jmlayman]

Yes - I am trying to import/export volumes programatically without using popen to invoke zpool. In the case of export (where it fails), I basically recreated what zpool_do_export() in zpool_main.c is doing in my code. Everything works great if the pool I am exporting is in /etc/mtab before my program starts because the logic inside zpool_disable_datasets() finds the pool in /etc/mtab & exports it. If I then import that pool (or another, for that matter) & then try to export it, the logic in zpool_disable_datasets() doesn't find the pool in /etc/mtab because it hasn't re-read it (it simply does a rewind). This causes zpool_disable_datasets() to fail & so the call to zpool_export() never happens. This all works fine when calling zpool import/export from the command-line because each invocation causes /etc/mtab to be re-read. However, since I am doing it programmatically the existing logic never re-reads /etc/mtab & so it never notices a volume has been imported. I suppose I could call libzfs_init() every time, but changing the logic to use freopen() seemed cleaner to me.