MINOR: config: report missing peers section name

Right now we report "invalid character ''" which is a bit confusing,
better make a special case of the missing name.
BUG/MEDIUM: tools: fix bad character handling in str2sa_range()

Commit d4448bc8 brought support for parsing port ranges, but invalid
characters are not properly handled and can result in a crash while
parsing the configuration if an invalid character is present in the
port, because the return value is set to NULL then dereferenced.
BUG/MEDIUM: ssl: ECDHE ciphers not usable without named curve configured.

Fix consists to use prime256v1 as default named curve to init ECDHE ciphers if none configured.