Makefile.config: fix/improve feature warnings.

[kolyshkin]

1. Tell which RPMs or DEBs are required in all cases.

2. Use $(info ...) everywhere.

3. Drop extra nested $(info), instead use (a document) a simpler kludge.

4. Simplify and unify the language, add missing periods.

Before:

Note: Building without setproctitle() and strlcpy() support.
      To enable these features, please install libbsd-devel (RPM) / libbsd-dev (DEB).
Note: Building criu without amdgpu_plugin.
Note: libdrm and libdrm_amdgpu are required to build amdgpu_plugin.
Makefile.config:53: Warn: you have no libnftables installed
Makefile.config:54: Warn: Building without nftables support

After:

Note: Building without setproctitle() and strlcpy() support.
      Install libbsd-devel (RPM) / libbsd-dev (DEB) to fix.
Note: Building without amdgpu_plugin.
      Install libdrm-devel (RPM) or libdrm-dev (DEB) to fix.
Warn: Building without nftables support.
      Install nftables-devel (RPM) or libnftables-dev (DEB) to fix.