Implement support for a SYS:ENV attribute on executable

__flatten_process_args() does now have the executable path as an
additional (optional) parameter. If specified, the function will read
the file's SYS:ENV attribute (if set) and use its value to modified the
environment it is preparing for the new process. Currently supported
attribute values are strings consisting of "<var>=<value>" substrings
separated by "\0" (backslash zero), with '\' being used as an escape
character. The environment will be altered to contain the specified
"<var>=<value>" elements, replacing a preexisting <var> element (if
any).

A possible use case would be setting a SYS:ENV attribute with value
"DISABLE_ASLR=1" on an executable that needs ASLR disabled.