lib: don't export all symbols, just everything curl_*

Absent any 'symbol map' or script to limit what gets exported, static
linking of libraries previously resulted in a libcurl with curl's and
those other symbols being (re-)exported.

This did not happen if 'versioned symbols' were enabled (which is not
the default) because then a version script is employed.

This limits exports to everything starting in 'curl_*'., which is
what "libcurl.vers" exports.

This avoids strange side-effects such as with mixing methods
from system libraries and those erroneously offered by libcurl.

Closes #2127