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
SQUASH ME: indentation level
SQUASH ME: fixed glue function
SQUASH ME: fixed more overdoings
SQUASH ME: one more missing glue function fixed
SQUASH ME: x86 rtc patch
SQUASH ME: another gluuuuue
SQUASH ME: more findings in native
Merge branch 'harmonize_pid' into ietf90

Conflicts:
	examples/ccn-lite-client/main.c
	sys/net/ccn_lite/ccn-lite-relay.c
	sys/net/ccn_lite/ccnl-core.h
ccnl: adapted to new thread_create API