Fix saving remotes with several fetch/push ref specs.

At some moment git_config_delete_entry lost the ability to delete one entry of
a multivar configuration. The moment you had more than one fetch or push
ref spec for a remote you will not be able to save that remote anymore. The
changes in network::remote::remotes::save show that problem.

I needed to create a new git_config_delete_multivar because I was not able to
remove one or several entries of a multivar config with the current API.
Several tries modifying how git_config_set_multivar(..., NULL) behaved were
not successful.

git_config_delete_multivar is very similar to git_config_set_multivar, and
delegates into config_delete_multivar of config_file. This function search
for the cvar_t that will be deleted, storing them in a temporal array, and
rebuilding the linked list. After calling config_write to delete the entries,
the cvar_t stored in the temporal array are freed.

There is a little fix in config_write, it avoids an infinite loop when using
a regular expression (case for the multivars). This error was found by the
test network::remote::remotes::tagopt.
Fix memory leaks.
Set new multivar values using unmatcheable regexp.

Seems that regexp in Mac OS X and Linux were behaving
differently: while in OS X the empty string didn't
match any value, in Linux it was matching all of them,
so the the second fetch refspec was overwritting the
first one, instead of creating a new one.

Using an unmatcheable regular expression solves the
problem (and seems to be portable).
Merge remote-tracking branch 'drodriguez/fix-remote-save' into development
config_file: Style fixes