git-svn: use POSIX::sigprocmask to block signals

git-svn: use POSIX::sigprocmask to block signals

In order to maintain consistency of the database mapping svn revision
numbers to git commit ids, rev_map_set() defers signal processing until
it's finished with an append transaction.[*]

The conventional way to achieve this is through sigprocmask(), which is
available in perl in the standard POSIX module.

This is implemented by this patch.  One important consequence of it is
that the signal handlers won't be unconditionally set to SIG_DFL anymore
upon the first invocation of rev_map_set() as they used to.  As a
result, the signals ignored by git-svn parent will remain ignored;
otherwise the behavior remains the same.

This patch paves the way to ignoring SIGPIPE throughout git-svn which
will be done in the followup patch.

[*] Deferring signals is not enough to ensure the database consistency:
the program may die on SIGKILL or power loss, run out of disk space,
etc.  However that's a separate issue that this patch doesn't address.

Signed-off-by: Roman Kagan <rkagan@mail.ru>
Acked-by: Eric Wong <normalperson@yhbt.net>