fix inconsistent use of strlen in jl_load

[stevengj]

I noticed that init.c was calling jl_load("boot.jl", sizeof("boot.jl")), which is wrong because the second argument is supposed to be the string length, but sizeof on a literal string is the string length + 1 to include the terminating NUL char.

Also, I noticed that the jl_parse_eval_all function wasn't actually using the len parameter in one place where it could have exploited it.

[stevengj]

Hmm, I also notice another bug: the jl_load and jl_parse_eval_all functions take a len parameter, but don't check for embedded NUL chars before passing filenames to filesystem routines expecting NUL-terminated data. Hence, they silently truncate filename strings containing NUL chars.

jl_load can certainly check for this, but I wonder if the flisp tostring function should also check?