recommend add support for lualatex

[Freed-Wu]

there exists a line of code

\ifnum\shellescape=1

in fact, if user use lualatex as their tex program, \shellescape does not exist. in replacement, it is \pdf@shellescape

so i recommend add condition judgment for different tex program.

it is an enhancement. thank you!

[muzimuzhi]

The names of read-only control sequences indicating the level of "shell escape" allowed vary between engines:

• xetex provides primitive \shellescape
• pdftex provides primitive \pdfshellescape
• luatex does not provide a similar primitive, but the package pdftexcmds defines \pdf@shellescape

Since latex2e release 2020-02-02, package expl3 is bundled into latex format. Hence I recommend use the uniform macro \c_sys_shell_escape_int defined by expl3 and documented in texdoc interface3, sec. XIV.7.