pwclient: allow per-directory settings

首先跟踪 pwclient 的入口点及其对 ~/.pwclientrc 的处理，然后比较两种拟议的配置方案：按项目进行目录匹配，或每个目录使用一个 .pwclientrc。所需的方案和匹配规则必须在实现之前确定；done 应包括自动选择项目，而无需显式的项目参数。

Currently, pwclient settings are stored in a single file (~/.pwclientrc), where multiple projects can be defined, but there is only one default.

[options]
default=linuxppc-dev

[linuxppc-dev]
url=https://patchwork.ozlabs.org/patchwork/xmlrpc/

[skiboot]
url=https://patchwork.ozlabs.org/patchwork/xmlrpc/

[snowpatch]
url=https://patchwork.ozlabs.ibm.com/patchwork/xmlrpc/ # Use our internal test patchwork

I use pwclient across a number of projects. pwclient should be able to figure out which project I'm looking at without me always having to explicitly specify it.

I'm thinking something along the lines of:

[options]
default=linuxppc-dev

[linuxppc-dev]
url=https://patchwork.ozlabs.org/patchwork/xmlrpc/
dir=/home/ajd/code/linux/

[skiboot]
url=https://patchwork.ozlabs.org/patchwork/xmlrpc/
dir=/home/ajd/code/skiboot*/ # Match a few different skiboot worktrees

[snowpatch]
url=https://patchwork.ozlabs.ibm.com/patchwork/xmlrpc/ # Use our internal test patchwork
dir=/home/ajd/code/snowpatch/

(the dir option is added)

Alternatively, a per-directory .pwclientrc, which could perhaps be distributed through the project's git repo.

[default]
project=snowpatch
url=https://patchwork.ozlabs.org/patchwork/xmlrpc/

(I haven't taken a good look at the pwclient source, so I haven't worked out how much work would be involved in doing this properly...)