vtimer: fix callback usage

There is no need to test the "handler" set in the vtimer struct, and
have some code executed then. We just can make the code to execute the
handler. To lengthy `if else if`, just a call.
core/msg: revert part of #1285

use m->sender_pid instead of sched_active_pid again
undoes the optimization intended by #1285
fixes uint/int warning
core/msg: set m->sender_pid before it is used

fixes `msg_send_to_self` condition
SQUASHME: do it right
make: move exported make vars to one place
make: don't use export on example makefiles
Merge pull request #1109 from Kijewski/make-exports

make: no "unfriedly" `exports` in the application's Makefile
Merge pull request #1387 from LudwigOrtmann/msg_fixup

core/msg: clean up after #1285
Merge pull request #1034 from Kijewski/vtimer-callbacks

vtimer: fix callback usage
core: harmonizes the data type for the process ID

Instead of using differing integer types use kernel_pid_t for process
identifier. This type is introduced in a new header file to avoid
circular dependencies.
SQUASH ME: cleanup commit - mostly formatters
SQUASH ME: revert changes to glue functions
SQUASH ME: fixed transceiver and fd
SQUASH ME: reverted some pid_t changes in native
SQUASH ME: fixed pid_t usage in fd and posix_io
SQUASH ME: removed now unnecessary and wrong cast