Description of problem:
1. -e signal=!io (example from manpage) is broken.
2. Missing bounds checks of user input cause segfaults.

Version-Release number of selected component (if applicable):
strace-4.5.8

Steps to Reproduce:
Actual results:

$ env -i strace -e 'signal=!io' /bin/true
strace: invalid signal `io'
$ strace -e signal=999999 true
Segmentation fault
$ strace -e read=999999 true
Segmentation fault


Expected results:
$ env -i strace -e 'signal=!io' /bin/true
execve("/bin/true", ["/bin/true"], [/* 0 vars */]) = 0
_exit(0)                                = ?
$ strace -e signal=999999 true
strace: invalid signal `999999'
$ strace -e read=999999 true
strace: invalid descriptor `999999'