An enhancement to GCC 10 to improve the expansion of strncmp
calls with strings with embedded nuls introduced a regression
in similar calls to memcmp.  A review of the changes that led
up to the regression exposed a number of questionable choices
that likely conspired to cause the bug.

For example, the name of the function with both the strncmp
enhancement as well as the memcmp bug is
inline_expand_builtin_string_cmp().  It's easy to assume that
the function handles calls to strcmp and strncmp but not also
memcmp.

Another similar example is the name of the second c_getstr()
argument -- strlen -- that doesn't actually store the length
of the retrieved string but rather its size in bytes
(including any embedded nuls, but excluding those appended
implicitly to zero out the remainder of an array the string
is stored in, up to the array's size).

Yet another example of a dubious choice is string_constant()
returning the empty string (i.e., STRING_CST with size 1) for
zero initializers of constants of any type (as opposed to one
of the same size as the constant object).

Besides fixing the memcmp bug the attached patch (hopefully)
also rectifies some of the otherwise more or less benign
mistakes that precipitated it, mostly by clarifying comments
and changing misleading names of functions, their arguments,
or local variables.

A happy consequence of the fixes is that they improve codegen
for calls to memcpy with constants whose representation includes
embedded nuls.

Tested on x86_64-linux.

Martin