env: fix shell function import

src/common/env.c:env_array_user_default() tries to import a "default"
user environment by parsing the output of something like

 su - <user> -c "/bin/env" ( or "slurmstepd getenv")

(or an equivalent) when the job is run with --get-user-env. There is some
code that tries to detect and import exported bash shell functions, which
usually have the form

 func=() { ...
 }

into the job environment. However, one of the many problems with the
code is that the test for bounds overflow:

              if ((strlen(value) + strlen(line)) >
                  (sizeof(value) - 1))
                     break;

useis sizeof(value) and value is a char *. This means that all multiline
shell functions are truncated, and the following error appears in
users batch script output:

 /bin/sh: func: line 1: syntax error: unexpected end of file
 /bin/sh: error importing function definition for `func'

This patch fixes the bounds test.

Similar cut-and-paste code in _load_env_cache() was also fixed.

However, it should be noted that there are many other potential cases
where shell functions will not be properly imported by this code.
(For example, quoted unbalanced brackets used in the function)
Someday this should be fixed.