Rename cv_wait_interruptible() to cv_wait_sig()

Rename cv_wait_interruptible() to cv_wait_sig()

Commit f752b46e added the cv_wait_interruptible() function to allow
condition variables to be woken by signals.  This function and its
timed wait counterpart should have been named cv_wait_sig() to match
the illumos interface which provides the same functionality.

This patch renames the symbol but leaves a #define compatibility
wrapper in place until the ZFS code can be moved to the correct
name.

This patch also makes a small number of cosmetic changes to make
the condvar source and header cstyle clean.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #456