execve.2: execve also returns E2BIG if a string is too long

The execve syscall returns -E2BIG in 3 cases:
- The total length of the command line arguments and environment is too large.
- An argument or environment string (including the NUL byte) is longer than MAX_ARG_STRLEN.
- The full path to the executable (including the NUL byte) exceeds MAX_ARG_STRLEN.

Spell out all 3 cases in the -E2BIG section.

Discovered by moving a too large commandline parameter to an environment
variable, and finding that things still did not work. Examined the code
in fs/exec.c to get the details.

This simple shell script starts failing at 2^17 on a system with 4kB
page size:
./exec2big.sh: line 10: /bin/true: Argument list too long
fork failed at loop 17

STRING="a"

for loop in `seq 20`; do
	STRING="$STRING$STRING"
	export STRING
	if /bin/true ; then
		: # still under the limit
	else
		echo "fork failed at loop $loop"
	fi
done

Signed-off-by: Rik van Riel <riel@surriel.com>
Suggested-by: Matthew House <mattlloydhouse@gmail.com>

man2/execve.2

@@ -449,7 +449,12 @@ The total number of bytes in the environment
 .RI ( envp )
 and argument list
 .RI ( argv )
-is too large.
+is too large,
+an argument or environment string is too long,
+or the full
+.I pathname
+of the executable is too long.
+The terminating NUL is counted as part of the string length.
 .TP
 .B EACCES
 Search permission is denied on a component of the path prefix of