package/libcurl: introduce options to extend/reduce feature set

Libcurl is more than 250 KiB (libcurl) / 100 KiB (curl binary) in size.
About 50 KiB / 15 KiB of this can be saved by disabling features/protocols
that are not commonly needed:

- proxy support: 15 KiB
- cookies support: 10 KiB
- various less common protocols: 25 KiB (libcurl) + 15 KiB (curl binary)

Note that the exact amount of space saved depends on the architecture,
toolchain, and other factors.

Other packages that are selecting libcurl might require protocols from the
'extra' set. But, there is no clear way to find out which packages are in
this situation, in particular because issues may only be visible at runtime.

Note: remove the text 'enable' on the option for 'verbose strings' as that
is more common in Buildroot.

Signed-off-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
[Peter: unconditionally remove the libcurl-option to generate C code]
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>