speed up exec() in ltlcross and ltldo

exec_with_timeout() could be speed up in at least two ways.

1. Try execv("/bin/sh",...) first and only resort to execvp("sh",...) if the first fails. This avoids the PATH lookup.

The first try should probably honor the SHELL variable if it is defined.

2. If the command is simple enough so that we can split it by ourselves (a single command with redirection, maybe single-quoted strings and double-quoted strings that do not contain dollars, backaslashs or backquotes, no variable assignment), then we can run the command directly with execvp without starting a shell. GNU make has some code that does exactly that.