Emulate /proc/self/exe when -q is used, and commit new value in execve() sysexit.

UMEQ (an awesome alternative to QEMU user-mode) does not emulate
"/proc/self/exe", moreover neither UMEQ nor QEMU can know how to
emulate "/proc/${pid}/exe", where ${pid} is different from self.

Also, new value of "/proc/self/exe" is committed only once execve(2)
is completed, that is, in exit stage.

Reported-by: Mickael GUENE <mickael.guene@st.com>