base-linux: ld.lib.so loads dynamic binaries

For ARM support on N900, commit 4a9b1c6 changed the process library to
start dynamic binaries directly depending on the Linux kernel to comply
to the interp section info ("ld.lib.so"). This seems not required on
more recent platforms or kernel versions and also introduced challenging
corner cases in region handling on Linux. Therefore, this commit
restores the original behavior.
base-linux: revised region management

Revised region management detects region conflicts by using _soft_
mappings per default. Overmapping is activated for population of managed
dataspaces only. For more information see header documentation of
base-linux/src/base/env/rm_session_mmap.cc.

Related to #883.
Move main bootstrap to platform-specific object

To prevent multiple execution of main-bootstrap, I moved the code to a
statically initialized object. The reason for this change is that
_main() is exeuted twice when starting dynamic binaries. Now, the object
is part of the base-common library which is linked with ld.lib.so.

Also fixes #883.