gnu: Add Aegis.

* gnu/packages/version-control.scm (aegis): New variable.
* gnu/packages/patches/aegis-constness-error.patch: New file.
* gnu/packages/patches/aegis-perl-tempdir1.patch: New file.
* gnu/packages/patches/aegis-perl-tempdir2.patch: New file.
* gnu/packages/patches/aegis-test-fixup-1.patch: New file.
* gnu/packages/patches/aegis-test-fixup-2.patch: New file.