calc: C-style arbitrary precision system Files

The following are the changes in this release:

    Fixed bug, when custom functions were enabled, where returning a
    value from the sysinfo custom function such as:

        custom("sysinfo","DEFAULTSHELL")

    would cause a segmentation violation due to an improper `free(3)`.

    Fixed a memory leak that was related to the sysinfo custom function.

    Added paranoia checks on shell value when processing commands
    read from the tty that start with the "!" character.

    Improved code that converts between UPPER CASE and lower case.

    Verify that the default underlying shell defaults to the shell
    as given by the `$SHELL` environment variable if that environment
    variable is set and is NOT empty, otherwise the `DEFAULTSHELL`
    (as defined in `calc.h`) value will be used as the underlying shell.

    Fixed memory leaks related to various environment variable values
    used by calc code.  All various environment variable values used
    used by calc code are processed by `strdup(3)`.

    The `libcalc_call_me_last()` function frees any non-NULL values set
    by `initenv()`.

    If the readline tilde_expand fails to expand "~/.calc_history",
    then ".calc_history" will be used as the name of the history file.

    Added HIST_NULL_HIST code to indicate that name of the history file
    could not be set.