Checkpoint on fixing compiling with musl gentoo

Restored use of the #define FPOS_POS_LEN symbol.  This refers to the
length of the __pos element (if the __pos element exists), the fpos_t type
(if that type exists).

Changed SWAP_HALF_IN_FILEPOS(dest, src) mato to use FILEPOS_LEN Little
Endian when FILEPOS is a simple scalar type (i.e., HAVE_FILEPOS_SCALAR
is defined).

Changed #define HAVE_FPOS to HAVE_FGETSETPOS.

Corrected comment in have_fpos_pos.h.  The HAVE_FPOS_POS symbol was NOT
related to fgetpos & fsetpos but rather if we we have an __pos element
in FILEPOS.

Changed #define HAVE_NO_FPOS to HAVE_NO_FGETSETPOS.

Improved comments in have_fpos_pos.h to show why and how HAVE_FPOS_POS,
FPOS_POS_BITS, and FPOS_POS_LEN were defined or undefed.

Changed showfiles builtin to print sizes cast to (long int) with the
"%ld" printf format to avoid implementations that do not support the
"%lld" printf format specifier.

file.c

@@ -2022,7 +2022,7 @@ showfiles(void)
 			math_chr('\n');
 			continue;
 		}
-		printf(" size = %lld\n", (long long int)sizes[i]);
+		printf(" size = %ld\n", (long int)sizes[i]);
 		for (j = i + 1; j < idnum; j++) {
 			if (listed[j] || sizes[j] == -1)
 				continue;