plugins: fix cygwin32 build

The isalpha() and isspace() compilation does fails with :

    array subscript has type ‘char’ [-Werror=char-subscripts]

There arg should be explicitely converted to int
node: cygwin does not have X_OK access mode
plugins: no need to redeclare environ

From the manpage :

    This variable must be declared in the user program, but is declared in the
    header file unistd.h in case the header files came from libc4 or libc5, and
    in case they came from glibc and _GNU_SOURCE was defined