Consolidate FIPS .hmac files

When FIPS integrity check is enabled, the library currently opens 4 .hmac files: libgnutls.so.* itself, libnettle.so.*, libhogweed.so.*, and libgmp.so.*. Given those files are part of individual packages (gnutls, nettle, and gmp), it is a bit hard to coordinate the package update. Therefore I propose consolidating those files into a single file, say .integrity with the following format:

[global]
format-version = ...

[integrity]
libgnutls = <hmac value>
libnettle = <hmac value>
libhogweed = <hmac value>
libgmp = <hmac value>

Or simply:

<hmac value> /usr/lib64/libgnutls.so.x
<hmac value> /usr/lib64/libnettle.so.x
<hmac value> /usr/lib64/libhogweed.so.x
<hmac value> /usr/lib64/libgmp.so.x

which should be mostly compatible with the previous format (except the trailing data after the first hmac value).

We may also consider embedding those information in libgnutls.so.* itself, using a similar technique proposed in integrity-notes.