Define constants as atoms

Instead of using a case statement, simplify the lookup by searching
through an array.

Do not modify the name of constants: previously, the names were
downcased and the leading namespace removed, e.g, PR_CAPBSET_READ
would become 'get_capbset_read' and SIG_TERM would be represented by
'term'.  Constants are now atoms mirroring the name in the header file:
'PR_CAPBSET_READ', 'SIG_TERM'.

This change will cause issues with non-portable naming of constants. For
example, Linux uses MS_ to preface mount constants and FreeBSD uses
MNT_. A future change will add portable representations of the atom.
Something like 'rdonly' for MS_RDONLY and MNT_RDONLY.
prctl: set length of binary in arg