alpha: enhance decoding of getxpid, getxuid, and getxgid syscalls

alpha: enhance decoding of getxpid, getxuid, and getxgid syscalls

Print the second return value of getxpid, getxuid, and getxgid syscalls
that return a pair of values using the same mechanism as pipe syscall.

* alpha.c: New file.
* Makefile.am (strace_SOURCES): Add it.
* linux/alpha/syscallent.h [20]: Change SEN(getpid) to SEN(getxpid).
[24]: Change SEN(getuid) to SEN(getxuid).
[47]: Change SEN(getgid) to SEN(getxgid).
* NEWS: Mention this enhancement.
* tests/uid.awk: Update for getxgid output change.
* tests/uid.test: Cleanup.
* tests/getxxid.c: New file.
* tests/getxxid.test: New test.
* tests/Makefile.am (check_PROGRAMS): Add getxxid.
(TESTS): Add getxxid.test.
* tests/.gitignore: Add getxxid.