Summary
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Add a new `-groupname` option to `keytool -genkeypair` so user can specify an named group when generating a keypair. For example, an Elliptic Curve name.

Problem
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In Elliptic Curve Cryptography, multiple curves can have the same field size (For example: both secp256r1 and brainpoolP256r1 are 256 bits). Therefore with only the `-keysize` option, there is no way to precisely specify which curve should be used.

Other key algorithms might have the same problem when multiple named groups have the same keysize.

Solution
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Add a new `-groupname` option to `keytool -genkeypair`. The user can specify a named group. Precisely:

 1. If both `-keysize` and `-groupname` are specified, there will be an error.
 1. If only `-groupname` is specified, the named group will be used.
 1. If only `-keysize` is specified, an arbitrary named group matching the specified size is used. For some algorithms (for example, EC) a warning will be printed suggesting migrating to `-groupname <the name>`.
 1. If neither is specified, the current default (for EC, it's secp256r1) will be used.

Specification
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In `keytool -help -genkeypair`, add one line

    -groupname <name>     Group name. For example, an Elliptic Curve name.

In keytool.html the tooldoc, adding the following paragraph to the `-genkeypair` command.

> The `-groupname` value specifies the named group (For example, the standard or predefined name of an Elliptic Curve) of the key to be generated. Only one of `-groupname` and `-keysize` can be specified.