Extract common example helpers and reorg examples

This reorganizes a few of the examples so that the main function
comes first with the argument parsing extracted into a helper
that can come at the end of the file (so the example focuses more
on the use of libgit2 instead of command line support).  This also
creates a shared examples/common.[ch] so that useful helper funcs
can be shared across examples instead of repeated.
init example: deploy helpers, reorg
add.c example: deploy helpers, reorg
init.c example: deploy more helpers
indexer: remove the stream infix

It was there to keep it apart from the one which read in from a file on
disk. This other indexer does not exist anymore, so there is no need for
anything other than git_indexer to refer to it.

While here, rename _add() function to _append() and _finalize() to
_commit(). The former change is cosmetic, while the latter avoids
talking about "finalizing", which OO languages use to mean something
completely different.
add.c: proper frontmatter
cat-file.c example: deploy helpers, reorg
rev-list.c example: use common utils, reorganize
Reorganize rev-parse example
Clean up showindex sample
Add tests for badly-formed URLs
Prevent segfault with a badly-formed URL
Test another bad URL
Prevent another segfault from bad URL
Format comments for use with docco
Update examples/README.md
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).
A few formatting changes for rocco

I'm not too happy about manually inserting &lt; and &gt; but those get
output as html tags otherwise.