[gentoo-catalyst] Fix the recent python sem_open failure

[Brian Dolbec]

These 2 patches I've just bumped up in the pending branch order.  They are needed
to fix recent changes to python build system.

The SOURCE_MOUNTS_DEFAULTS is to compliment the TARGET_MOUNTS_DEFAULTS and
simplifies it's use through the code.  It also is one step closer to having them 
configurable.

The shm patch is a merge of 3 patches into this final one.
The changes have all been tested.   These jsut need testing due to the rebase.

I hope to merge these patches this weekend along with teh previous 6 that fix
other commits already applied.  They have had lots of scrutiny and review already.