Fix some #! misparsing in check_interpreter

Fix some #! misparsing in check_interpreter

* exec/exec.c: Do not include ctype.h, as the kernel
does not care about the locale.
(check_interpreter): Treat only spaces and tabs as white space.
Do not inspect more bytes than were read.
Although the resulting code does not exactly match what
the Android kernel does, it’s closer than what it was before.

(cherry picked from commit e0b027d1215ed32fe0f3d0956d2d7a81552274f2)