tools/version.sh: Update embedded version numbers in build products

We can now use `tools/version.sh --release` to update the MANIFEST.MF
and Maven POM files with the current version number of this project,
so they appear in any build product created.

The counterpart --snapshot option be used to reset files to use
their natural *-SNAPSHOT and *.qualifier state during development.

We use a simple Bourne shell script with Perl calls because we
must edit both Maven pom.xml and OSGi bundle MANIFEST.MF in order
to store the correct data for our parallel build systems.  In the
future we should use a native Java solution which relies upon JGit
to compute the `git describe` portion.

Until we tag our first official release a "tagged snapshot" can be
made by creating an artifical annotated tag first:

  git tag -a -m "initial contribution" v0.5.1 046198cf5f21e5a63e8ec0ecde2ef3fe21db2eae
  tools/version.sh --release

Resulting in a version string like "0.5.1.50-ge16af83".

Change-Id: Ic2bbae75bf96fc8831324c62c2212131277f70e4
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>