Revert "rexec: remove needless /proc/cmdline parsing"

Revert "rexec: remove needless /proc/cmdline parsing"

The ELF binary spec does specify in [1]:

SHT_INIT_ARRAY
This section contains an array of pointers to initialization functions,
as described in ``Initialization and Termination Functions'' in Chapter
5. Each pointer in the array is taken as a parameterless procedure with
a void return.

which means libcs other than glibc might not pass down argc and argv to
constructors.

This reverts commit 0c816b346788afa9d601766e31544fdcce67d780.

[1]: http://www.sco.com/developers/gabi/latest/ch4.sheader.html#init_array

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>