Configuration: New check to see if /dev/shm allows execution

On systems that allow it (like GNU/Linux systems), Maneage will build the
necessary software in shared memory (a directory that is actually in the
RAM, not on an SSD/HDD, on GNU/Linux systems, it is '/dev/shm'). This
allows Maneage to operate faster and not harm the HDD/SSD with all the
temporary writing of many small files.

Until now, we would only check that this directory exists and that it has
enough space. However, some systems also set the 'noexec' flag on shared
memory for security reasons [1]. This causes Maneage to crash upon building
of the software in later phases.

With this commit, at the very start of the configuration step, and after
all other shared-memory checks are done, a dummy executable script file is
created there and its execution is tested. If it doesn't work, shared
memory will not be used at all.

In the process, the steps dealing with the software building directory in
the configure script have been brought in one place and comments were added
to further clarify every step.

This commit was initially done by Boud Roukema and later edited by Mohammad
Akhlaghi.

[1] https://web.archive.org/web/20210624192819/https://serverfault.com/questions/72356/how-useful-is-mounting-tmp-noexec

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